System and methods for matching electrical characteristics and propagation velocities in cardiac tissue

ABSTRACT

Systems and methods examine heart tissue morphology using three or more spaced apart electrodes, at least two of which are located within the heart in contact with endocardial tissue. The systems and methods transmit electrical current through a region of heart tissue lying between selected pairs of the electrodes, at least one of the electrodes in each pair being located within the heart. The systems and methods derive the electrical characteristic of tissue lying between the electrode pairs based, at least in part, upon sensing tissue impedances. The systems and methods also sense the timing of local depolarization events in the tissue in which impedance is sensed and derive therefrom the propagation velocities of the sensed depolarization events. The systems and methods match the derived tissue electrical characteristics with the derived propagation velocities in spatial relation to the electrodes to characterize the morphology of the contacted heart tissue. This matching provides precise differentiation between regions of infarcted heart tissue and healthy heart tissue.

RELATED APPLICATION

This application claims the benefit of prior filed copending U.S. patentapplication Ser. No. 08/188,242, Jan. 28, 1994, entitled "Systems andMethods for Deriving and Displaying the Propagation Velocities ofElectrical Events in the Heart."

1. Field of the Invention

The invention relates to systems and methods for mapping the interiorregions of the heart for treatment of cardiac conditions.

2. Background of the Invention

Physicians examine the propagation of electrical impulses in hearttissue to locate aberrant conductive pathways. The aberrant conductivepathways constitute peculiar and life threatening patterns, calleddysrhythmias. The techniques used to analyze these pathways, commonlycalled "mapping," identify regions in the heart tissue, called foci,which are ablated to treat the dysrhythmia.

Conventional cardiac tissue mapping techniques use multiple electrodespositioned in contact with epicardial heart tissue to obtain multipleelectrograms. Digital signal processing algorithms convert theelectrogram morphologies isochronal displays, which depict thepropagation of electrical impulses in heart tissue over time. Theseconventional mapping techniques require invasive open heart surgicaltechniques to position the electrodes on the epicardial surface of theheart.

Furthermore, conventional epicardial electrogram processing techniquesused for detecting local electrical events in heart tissue are oftenunable to interpret electrograms with multiple morphologies. Suchelectrograms are encountered, for example, when mapping a heartundergoing ventricular tachycardia (VT). For this and other reasons,consistently high correct foci identification rates (CIR) cannot beachieved with current multi-electrode mapping technologies.

Researchers have taken epicardial measurements of the electricalresistivity of heart tissue. Their research indicates that theelectrical resistivity of infarcted heart tissue is about one-half thatof healthy heart tissue. Their research also indicates that ischemictissue occupying the border zone between infarcted tissue and healthytissue has an electrical resistivity that is about two-thirds that ofhealthy heart tissue. See, e.g., Fallert et al., "Myocardial ElectricalImpedance Mapping of Ischemic Sheep Hearts and Healing Aneurysms,"Circulation, Vol. 87, No. 1, January 1993, 199-207.

This observed physiological phenomenon, when coupled with effective,non-intrusive measurement techniques, can lead to cardiac mappingsystems and procedures with a CIR better than conventional mappingtechnologies.

SUMMARY OF THE INVENTION

A principal objective of the invention is to provide improved probes andmethodologies to examine heart tissue morphology quickly, accurately,and in a relatively non-invasive manner.

One aspect of the invention provides systems and methods for examiningheart tissue morphology using at least three spaced apart electrodes, atleast two of which are located in contact with a region of heart tissue.The systems and methods transmit electrical current in a first paththrough the region of heart tissue between a first pair of theelectrodes. The systems and methods transmit electrical current in asecond path through heart tissue in the region between a second pair ofthe electrodes. The systems and methods derive from the electricalcurrent transmissions tissue electrical characteristics based, at leastin part, upon sensing the impedances of the tissue lying in the firstand second paths.

This electrical characteristic (called the "E-Characteristic") can bedirectly correlated to tissue morphology. A low relativeE-Characteristic indicates infarcted heart tissue, while a high relativeE-Characteristic indicates healthy heart tissue. IntermediateE-Characteristic values indicate the border of ischemic tissue betweeninfarcted and healthy tissue.

How the E-Characteristic is expressed depends upon how the electricalcurrent is transmitted by the electrode pair through the heart tissue.

When one of the electrodes in the pair comprises an indifferentelectrode located outside the heart (i.e., a unipolar arrangement), theE-Characteristic is expressed in terms of tissue impedance (in ohms).When both electrodes in the pair are located inside the heart (i.e., abipolar arrangement), the E-Characteristic is expressed in terms oftissue resistivity (in ohm-cm).

The systems and methods also sense the timing of local depolarizationevents in the tissue in which impedance is sensed and derive therefromthe propagation velocities of the sensed depolarization events. Thesystems and methods match the derived tissue electrical characteristicswith the derived propagation velocities in spatial relation to theelectrodes to characterize the morphology of the contacted heart tissue.

This matching in effect matches electrogram morphologies with tissueE-Characteristics. This matching provides more precise differentiationbetween regions of infarcted heart tissue and healthy heart tissue thanconventional systems and methodologies.

In a preferred embodiment, the systems and methods normalize the derivedtissue electrical characteristics and the derived propagation velocitiesin spatial relation to the electrodes. In this embodiment, the systemsand methods match the normalized derived electrical characteristics withthe normalized derived propagation velocities in spatial relation to theelectrodes.

In a preferred embodiment, the system and method apply two dimensionalmatched filtering to the normalized derived tissue electricalcharacteristics using the normalized propagation velocities as atemplate, or vice versa.

In a preferred embodiment, the matched values range in value in spatialrelation to the electrodes. In this embodiment, systems and methodsnormalize the value range to a normalized range of 0.0 to 1.0 in spatialrelation to the electrodes by dividing each matched value in the valuerange by the maximum matched value in the value range. In thisembodiment, the systems and methods output the normalized matched valuesin groups of equal matched values in spatial relation to the electrodes.

These normalized matching techniques make possible the mapping of hearttissue to aid in the identification of possible tissue ablation sites.

Other features and advantages of the inventions are set forth in thefollowing Description and Drawings, as well as in the appended Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view, with portions in section, of a system forexamining and mapping heart tissue morphology according to the featuresof the invention, shown deployed for use within the heart;

FIG. 2 is a plan view, with portions in section, of the system shown inFIG. 1 in the process of being deployed for use within the heart;

FIG. 3 is a view of the mapping probe and process controller associatedwith the system shown in FIG. 1;

FIG. 4 is an enlarged perspective view of an electrode carrying splineassociated with the probe shown in FIG. 1;

FIG. 5 is a cross sectional view of an alternative embodiment of anelectrode that can be associated with the probe shown in FIG. 1, takengenerally along line 5--5 in FIG. 6;

FIG. 6 is an enlarged perspective view of an alternative embodiment ofan electrode carrying spline that can be associated with the probe shownin FIG. 1;

FIGS. 6A to 6C and associated catheter tube are views of a flexibleelectrode support body that can carry the electrodes and deployed in theheart according to the invention;

FIG. 7 is a schematic view of the current generator module and switchingelement of the process controller for the system shown in FIG. 1;

FIG. 8 is a diagrammatic view of the current generator module andswitching element when operated in a Unipolar Mode;

FIG. 9 is a diagrammatic view of the current generator module andswitching element when operated in a Bipolar Two Electrode Mode;

FIG. 10 is a diagrammatic view of the current generator module andswitching element when operated in a Bipolar Four Electrode Mode;

FIGS. 11 and 12 are schematic views of the details of the switchingelement shown in FIGS. 7 to 10;

FIG. 13 is a schematic view of the signal processor module of theprocess controller for the system shown in FIG. 1;

FIG. 14 is a schematic view of the E-Characteristic computing system ofthe signal processor module shown in FIG. 13;

FIG. 15 is an illustrative, idealized display of the absolute tissueE-Characteristic values derived by the system shown in FIG. 14 arrangedin spatial relation to a region of the heart;

FIG. 16 is a flow chart showing the operation of the system thatarranges the derived absolute tissue E-Characteristic values into groupsof equal values;

FIG. 17 is a representative display of the groups of equalE-Characteristic values derived by the system shown in FIG. 16 arrangedin spatial relation to a region of the heart; and

FIG. 18 is a diagrammatic view of an alternative embodiment of acontroller that can be used in association with the system shown in FIG.1;

FIG. 19 is a diagrammatic view of the pacing module that the controllershown in FIG. 18 includes;

FIG. 20 is a diagrammatic view of the host processing unit andelectrogram signal processing module with which the controller shown inFIG. 18 is associated;

FIG. 21A is a view of four representative electrograms that can be usedto compute electrogram events;

FIG. 21B is a flow chart showing the methodology for computing anelectrogram event for processing by the controller shown in FIG. 18;

FIG. 22 is a flow chart showing the operation of the means forconstructing an iso-display of the computed electrogram event;

FIG. 23 is a representative iso-chronal display;

FIG. 24 is a flow chart showing the operation of the means forconstructing an iso-conduction display of the computed electrogramevent;

FIG. 25 is a representative iso-conduction display;

FIG. 26 is a flow chart showing the operation of the means for matchingiso-E-Characteristics with iso-conduction information;

FIG. 27 is a representative display of the matched iso-E-Characteristicsand iso-conduction information;

FIG. 28 is a flow chart showing the operation of the means for detectinga possible ablation site based upon the information obtain in FIG. 26;

FIG. 29 is a representative display of the matched Iso-E-Characteristicsand iso-conduction information, after selection of a threshold value,identifying a potential ablation site; and

FIG. 30 is a plan view of an ablation probe being used in associationwith the system shown in FIG. 1.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 3 show the components of a system 10 for examining hearttissue morphology. FIG. 1 shows the system 10 deployed and ready for usewithin a selected region 12 inside a human heart.

As FIGS. 1 and 2 show, deployment of the system 10 does not requireinvasive open heart surgical techniques. Instead, the system 10 includesan introducer 14 and an outer guide sheath 16 that together direct amultiple electrode probe 18 into the selected region 12 within the heartthrough a selected vein or artery. FIG. 3 shows the probe 18 in itsentirety.

The physician uses the probe 18 in association with a process controller20 (see FIG. 3) to take multiple, sequential measurements of thetransmission of electrical current by heart tissue. From these, theelectrical characteristic of the tissue is derived.

This electrical characteristic (called the "E-Characteristic") can bedirectly correlated to tissue morphology. A low relativeE-Characteristic indicates infarcted heart tissue, while a high relativeE-Characteristic indicates healthy heart tissue. IntermediateE-Characteristic values indicate the border of ischemic tissue betweeninfarcted and healty tissue.

In the illustrated and preferred embodiment, these measurements are usedto assist the physician in identifying appropriate ablation sites withinthe heart.

How the E-Characteristic is expressed depends upon how the electricalcurrent is transmitted by an electrode pair through the heart tissue.

When one of the electrodes in the pair comprises an indifferentelectrode located outside the heart (i.e., a unipolar arrangement), theE-Characteristic is expressed in terms of tissue impedance (in ohms).When both electrodes in the pair are located inside the heart (i.e., abipolar arrangement), the E-Characteristic is expressed in terms oftissue resistivity (in ohm-cm).

FIG. 1 and the other figures generally show the system 10 deployed inthe left ventricle of the heart. Of course, the system 10 can bedeployed in other regions of the heart, too. It should also be notedthat the heart shown in the Figures is not anatomically accurate. TheFigures show the heart in diagrammatic form to demonstrate the featuresof the invention.

I. NON-INVASIVE SYSTEM DEPLOYMENT

As FIG. 1 shows, the introducer 14 has a skin-piercing cannula 22. Thecannula 22 establishes percutaneous access into the selected vein orartery (which is typically the femoral vein or artery). The other end ofthe introducer 14 includes a conventional hemostatic valve 24.

The physician advances the outer guide sheath 16 through the introducer14 through the vein or artery into the selected heart chamber 12. Thehemostatic valve 24 yields to permit the introduction of the outer guidesheath 16 through it, but otherwise conforms about the outer surface ofthe sheath 16, thereby maintaining a fluid tight seal.

Preferably, the guide sheath 16 includes a precurved distal tip region26, like a conventional "pig tail" catheter. The precurved distal tipregion 26 assists in steering the guide sheath 16 into position withinthe heart chamber 12.

The physician advances the probe 18 through the handle 28 of the outersheath 16. The handle 28 includes a second conventional hemostatic valve30 that yields to permit the introduction of the flexible body 32 of themapping probe 18 through it. At the same time, the valve 30 conformsabout the outer surface of the body 22 to maintain a fluid tight seal.

Further details of the deployment and use of the introducer 14 and guidesheath 16 to establish a pathway for the probe 18 are set forth inpending U.S. patent application Ser. No. 08/033,641, filed Mar. 16,1993, entitled "Systems and Methods Using Guide Sheaths for Introducing,Deploying, and Stabilizing Cardiac Mapping and Ablation Probes."

II. THE TISSUE EXAMINATION PROBE

As FIGS. 1 and 3 best show, the probe 18 has a handle 34 attached to theproximal end of the flexible catheter body 32. The distal end of thecatheter body 32 carries a three dimensional structure 36. In FIGS. 1and 3, the structure 36 takes the form of a basket. It should beappreciated that other three dimensional structures could be used.

The three dimensional basket structure 36 carries an array of electrodes38.

As FIG. 1 shows, when deployed inside the heart chamber 12, the basketstructure 36 holds the electrodes 38 in intimate contact against theendocardial surface of the heart chamber 12.

The catheter body 32 passes through the outer guide sheath 16. Thesheath 16 has an inner diameter that is greater than the outer diameterof the catheter body 32. As a result, the sheath 16 can slide along thecatheter body 32. The sheath handle 28 helps the user slide the sheath16 along the catheter body 32.

As FIG. 2 shows, forward movement of the sheath handle 28 (i.e., towardthe introducer 14) advances the distal end of the slidable sheath 16upon the basket structure 36. In this position, the slidable sheath 16captures and collapses the basket structure 36, entirely enclosing thebasket structure 36.

As FIG. 1 shows, rearward movement of the sheath handle 28 (i.e., awayfrom the introducer 14) retracts the slidable sheath 16 away from thebasket structure 36. This removes the compression force, and the basketstructure 36 opens to assume its prescribed three dimensional shape.

The probe 18 also preferably includes a sliding hemostat sheath 40. Thephysician slides the sheath 40 about the basket structure 36 to protectit during its advancement through the introducer 14. Once the basketstructure 36 enters the guide sheath 16, the physician slides thehemostatic sheath 40 away rearward toward the probe handle 34. Furtherdetails of the use of the sheath 40 are disclosed in theabove-identified pending Patent Application.

The basket structure 36 can itself be variously constructed. In theillustrated and preferred embodiment (see FIG. 3), the basket structure36 comprises a base member 42 and an end cap 44. Generally flexiblesplines 46 extend in a circumferentially spaced relationship between thebase member 42 and the end cap 44.

In the illustrated embodiment, eight, rectilinear splines 46 form thebasket structure 36. However, additional or fewer splines 46 could beused, as could spline of different configurations.

In this arrangement, the splines 46 are preferably made of a resilientinert material, like Nitinol metal or silicone rubber. The splines 46are connected between the base member 42 and the end cap 44 in aresilient, pretensed condition, shown in FIG. 3.

As FIG. 1 shows, the resilient splines 46 bend and conform to theendocardial tissue surface they contact. As FIG. 2 shows, the splines 46also collapse into a closed, compact bundle in response to the externalcompression force of the sliding sheath 18.

In the illustrated embodiment (see FIG. 4), each spline 46 carries eightelectrodes 38. Of course, additional or fewer electrodes 38 can be used.

As will be described later, the system 10 can be operated in either aunipolar mode or a bipolar mode. The basket electrodes 38 can thereforebe arranged in thirty-two bi-polar pairs, or as sixty-four uni-polarelements.

In the illustrated and preferred embodiment (as FIG. 4 best shows), theelectrodes 38 are mounted to each spline 46 to maximize surface contactto endocardial tissue, while at the same time minimizing exposure to thesurrounding blood pool. Incidental exposure of the electrodes 38 toblood while in contact with heart tissue introduces an unwanted artifactto E-Characteristic measurement, because the resistivity of blood isabout three times lower than the resistivity of heart tissue. Thisartifact can skew the E-Characteristic measurement to a lower value,thereby reducing the desired contrast between healthy and infarctedtissue.

In the preferred embodiment (see FIG. 4), the electrodes 38 are made ofplatinum or gold plated stainless steel bands affixed to only one sideof the splines 46. This is the side of the spline 46 that, in use,contacts endocardial tissue. The opposite surface of the splines 46(which, in use, contacts the blood pool) is free of electrodes.

In an alternative arrangement (see FIGS. 5 and 6), the electrodes 38 cantake the form of rings that encircle the entire spline 46. In thisarrangement, the rear side of the electrodes 38, which during use facethe blood pool, are coated with an electrically insulating material 49to prevent current transmission into blood.

It is believed that no more than 20% of the electrode surface should beexposed to the blood pool during use. Preferable, less than 5% of theelectrode should be so exposed during use.

In an alternative arrangement (see FIGS. 6A to 6C), one or more ofelectrodes 38 can be introduced into the heart chamber through a vein orartery on a single flexible electrode support body 300, and not on abasket structure like that earlier described. The body 300 isillustrative of a family of flexible, elongated electrode supports ofvarious alternative constructions. In the preferred and illustratedembodiment, the body 300 is about 1 to 2.5 mm in diameter and about 1 to5 cm long.

As FIG. 27 shows, the body 300 is carried at the distal end of acatheter tube 302 used to guide the body 300 into the heart. A handle304 is attached to the proximal end of the catheter tube 302. The handle304 and catheter tube 302 carry a steering mechanism 306 for selectivelybending or flexing the support body 300 along its length, as the arrowsin FIG. 6A show.

The steering mechanism 306 can vary. In the illustrated embodiment (seeFIG. 6C), the steering mechanism 306 includes a rotating cam wheel 308with an external steering lever 310 (as FIG. 6A shows). As FIG. 6Cshows, the cam wheel 308 holds the proximal ends of right and leftsteering wires 312. The wires 312 pass through the catheter tube 302 andconnect to the left and right sides of a resilient bendable wire orspring (not shown) within the ablating element support body 300.

As FIG. 6A shows, movement of the steering lever 310 flexes or curvesthe support body 300 from a generally straight configuration (shown inphantom lines in FIGS. 6A and 6B) into a generally arcuate curve (shownin solid lines in FIGS. 6A and 6B). Through flexing, the electrodes 38can also be brought into conforming, intimate contact against theendocardial tissue, despite the particular contours and geometry thatthe wall presents.

As shown in FIG. 6B, the electrodes 38 comprise rings encircling thesupport body 300. In this arrangement, the rear sides of the electrodes38, which, in use, face the blood pool, are preferably coated with theelectrical insulating material 49 for the reasons stated above.Alternatively, the electrodes 38 can be affixed only to the tissuecontacting side of the support body 300, thereby making the rear side ofthe support body 300 free of electrodes 38, like the rectilinear spline46 shown in FIG. 4.

The electrodes 38 carried by the support body 300, as FIG. 6B shows, canby used in association with the process controller 20 to take one ormore E-Characteristic measurements, just as the electrodes carried bythe basket structure. The support body 300 can be moved sequentially todifferent endocardial sites to obtain a plurality of E-Characteristicmeasurements, which can be processed in the same manner as those takenby the stationary basket structure.

Further details of flexible electrode carrying elements can be found incopending U.S. patent application Ser. No. 08/138,142, filed Oct. 15,1993, entitled "Systems and Methods for Creating Long, Thin Lesions inBody Tissue."

In the illustrated embodiments (see FIGS. 4 and 6), a signal wire 47made from a highly conductive metal, like copper, leads from eachelectrode 46 (these signal wires are also shown diagrammatically in FIG.11). The signal wires 47 extend down the associated spline 46, by thebase member 42, and into the catheter body 32. An inert plastic wrapping43 preferably covers each spline 46 and electrode support body 300,except where the electrodes 38 project, to shield the signal wires.

The eight signal wires 47 for each spline 46 are twisted together toform a common bundle. The eight common bundles (not shown) are, in turn,passed through the catheter body 32 of the mapping probe 18. The commonbundles enter the probe handle 34.

The sixty-four signal wires 47 are connected within the probe handle 34to one or more external connectors 48, as FIG. 3 shows. In theillustrated embodiment, each connector contains thirty-two pins toservice thirty-two signal wires.

In an alternative arrangement (not shown), the electrodes 38 can beconnected to a multiplexer/demultiplexer (M/DMUX) block (not shown) toreduce the number of signal wires carried by the catheter body 32. TheM/DMUX block can comprise a multi-die integrated circuit mounted on aflexible support and wrapped about the catheter body 32. Thesignal-to-noise-ratio is thereby improved.

III. MEASURING AND MAPPING THE TISSUE E-CHARACTERISTIC

The system 10 transmits electrical current in a selected manner throughthe basket electrodes 38 in contact with endocardial tissue. From this,the system 10 acquires impedance information about the heart tissueregion that the basket electrodes 38 contact. The system 10 processesthe impedance information to derive the E-Characteristic, which assiststhe physician in identifying regions of infarcted tissue where ablationtherapy may be appropriate.

For these purposes (see FIG. 3), the system 10 includes the processcontroller 20. The process controller 20 includes a current generatormodule 50 and a signal processor module 52. The connectors 48electrically couple the basket electrodes 38 to both the generatormodule 50 and the processor module 52.

A. The Current Generator Module

The generator module 50 conveys a prescribed current signal toindividual basket electrodes 38.

In the illustrated and preferred embodiment (see FIG. 7), the generatormodule 50 includes an oscillator 54 that generates a sinusoidal voltagesignal. An associated interface 56 has a bus 58 that controls thefrequency of the output voltage signal and a bus 60 that controls theamplitude of the output voltage signal. The interface 56, in turn, isprogrammed by a host processor 206, which will be described in greaterdetail later.

The oscillator 54 has as an output stage that includes avoltage-to-current converter 62. In conventional fashion, the converter62 converts the sinusoidal voltage signal to current.

In the illustrated and preferred embodiment, the transmitted current hasan amplitude of about 0.1 milliamps to 5.0 milliamps. The lower range ofthe current amplitude is selected to be high enough to overcome theinfluence of the double layer at the tissue-electrode interface on theE-Characteristic measurement. The high range of the current amplitude isselected to avoid the induction of fibrillation.

The current has a frequency in a range of about 5 to 50 kHz. The rangeis selected to avoid the induction of fibrillation, as well as providecontrast between infarcted tissue and healthy tissue. The output of theconverter 62 can comprise a constant current with a constant frequencywithin the above range. Alternatively, the interface 56 can control themodulation of the frequency of the current signal within the prescribedrange. Deriving tissue E-Characteristic by transmitting currents withdifferent frequencies better differentiates among different tissuemorphologies. It has been determined that lower frequencies within therange provide E-Characteristics yielding greater quantitative contrastbetween infarcted and healthy tissues than higher frequencies in thisrange.

The current output of the module 50 is supplied to the basket electrodes38 via supply path 68 through a switching element 64. The interface 56electronically configures the switching element 64 to direct current insuccession to selected basket electrodes 38 through their associatedsignal wires in either a unipolar mode or a bipolar mode. Line 66constitutes the control bus for the switching element 64.

As FIG. 8 shows, when operated in a unipolar mode, the current returnpath 70 to the generator module 50 is provided by an exteriorindifferent electrode 72 attached to the patient.

When operated in a bipolar mode, the current return path 70 is providedby an electrode carried on the basket structure 36 itself. In theillustrated and preferred embodiment, the bipolar return electrode iseither located immediately next to or three electrodes away from theselected transmitting basket electrode along the same spline. The firstcircumstance (shown in FIG. 9) will be called the Bipolar Two ElectrodeMode. The second circumstance (shown in FIG. 10) will be called theBipolar Four Electrode Mode.

The configuration of the switching element 64 can vary. FIG. 11diagrammatically shows one preferred arrangement.

FIG. 11 shows for illustration purposes a spline 46 with seven adjacentelectrodes 38, designated E1 to E7. Each electrode E1 to E7 iselectrically coupled to its own signal wire, designated W1 to W7. Theindifferent electrode, designated EI in FIG. 11, is also electricallycoupled to its own signal wire WI.

In this arrangement, the switching element 64 includes an electronicswitch S_(M) and electronic switches S_(E1) to S_(E7) that electricallycouple the current generator to the signal wires W1 to W7. The switchS_(M) governs the overall operating mode of the electrodes E1 to E7(i.e., unipolar or bipolar). The switches S_(E1) to S_(E7) govern theelectrical conduction pattern of the electrodes E1 to E7.

The switches S_(M) and S_(E1) to E7 are electrically coupled to thecurrent source. The supply path 68 of the generator module 50 iselectrically coupled to the leads L1 of the switches S_(E1) to E7. Thereturn path 70 of the generator module 50 is electrically coupled to thecenter lead L2 of the mode selection switch S_(M). A connector 67electrically couples the leads L3 of the switches S_(M) and S_(E1) toE7.

The center leads L2 of the selecting switches S_(E1) to E7 are directlyelectrically coupled to the signal wires W1 to W7 serving the electrodesE1 to E7, so that one switch S_(E)(N) serves only one electrodeE.sub.(N).

The lead L1 of the switch S_(M) is directly electrically coupled to thesignal wire WI serving the indifferent electrode EI.

The interface 56 electronically sets the switches S_(M) and S_(E1) to E7among three positions, designated A, B, and C in FIG. 12.

As FIG. 12 shows, Position A electrically couples leads L1 and L2 of theassociated switch. Position C electrically couples leads L2 and L3 ofthe associated switch. Position B electrically isolates both leads L1and L3 from lead L2 of the associated switch.

Position B is an electrically OFF position. Positions A and B areelectrically ON positions.

By setting switch S_(M) in Position B, the interface 56 electronicallyinactivates the switching network 54.

By setting switch S_(M) in Position A, the interface 56 electronicallyconfigures the switching element for operation in the unipolar mode. Thecenter lead L2 of switch S_(M) is coupled to lead L1, electronicallycoupling the indifferent electrode EI to the return of the currentgenerator. This configures the indifferent electrode EI as a return pathfor current.

With switch S_(M) set in Position A, the interface 56 electronicallyselectively configures each individual electrode E1 to E7 to emitcurrent by sequentially setting the associated switch S_(E1) to E7 inPosition A. When the selected electrode E1 to E7 is so configured, it iselectronically coupled to the supply of the current generator and emitscurrent. The indifferent electrode EI receives the current sequentiallyemitted by the selected electrode E1 to E7.

By setting switch S_(M) in Position C, the interface 56 electronicallyisolates the indifferent electrode EI from the electrodes E1 to E7. Thisconfigures the switching element for operation in the bipolar mode.

With switch S_(M) set in Position C, the interface 56 can electronicallyalter the polarity of adjacent electrodes E1 to E7, choosing amongcurrent source, current sink, or neither.

By setting the selected switch S_(E1) to E7 in Position A, the interface56 electronically configures the associated electrode E1 to E7 to be acurrent source. By setting the selected switch S_(E1) to E7 in PositionC, the interface 56 electronically configures the associated electrodeE1 to E7 to be a current sink. By setting the selected switch S_(E1) toE7 in Position B, the interface 56 electronically turns off theassociated electrode E1 to E7.

In the Bipolar Two Electrode Mode, the interface 56 first configures theelectrode E1 to be a current source, while configuring the immediateadjacent electrode E2 to be a current sink, while turning off theremaining electrodes E3 to E7. After a preselected time period, theinterface 56 then turns off electrode El, configures electrode E2 to bea current source, configures the next immediate adjacent electrode E3 tobe a current sink, while keeping the remaining electrodes E4 to E7turned off. After a preselected time period, the interface 56 then turnsoff electrode E2, configures electrode E3 to be a current source,configures the next immediate adjacent electrode E4 to be a currentsink, while keeping the remaining electrodes E1 and E5 to E7 turned off.The interface 56 cycles in this timed sequence until electrodes E6 andE7 become the current source/sink bipolar pairs (the remainingelectrodes E1 to E5 being turned off). The cycle can then be repeated,if desired, or ended after one iteration.

In the Bipolar Four Electrode Mode, the interface 56 first configuresthe electrode E1 to be a current source, while configuring the thirdadjacent electrode E4 to be a current sink, while turning off theremaining electrodes E2, E3, and E5 to E7. After a predetermined timeperiod, the interface 56 turns off electrode El, configures electrode E2to be a current source, configures the next third adjacent electrode E5to be a current sink, while keeping the remaining electrodes E3, E4, E6,and E7 turned off. After a predetermined time period, the interface 56turns off electrode E2, configures electrode E3 to be a current source,configures the next third adjacent electrode E6 to be a current sink,while keeping the remaining electrodes E1, E2, E4, E5, and E7 turnedoff. The interface 56 cycles in this timed sequence until electrodes E4and E7 become the current source/sink bipolar pairs (the remainingelectrodes E1 to E3, E5, and E6 being turned off. The cycle can then berepeated, if desired, or ended after one iteration.

In the preferred embodiment, there is a switching element 64 for theelectrodes on each basket spline, with the interface 56 independentlycontrolling each switching element.

B. Computing Tissue E-Characteristic

As FIG. 13 shows, the signal processor module 52 includes a dataacquisition system While current is emitting by a selected basketelectrode, the system 74 senses the voltage in the tissue path usingselected electrodes on the basket 36.

Based upon the data acquired by the system 74, the host processor 206computes the E-Characteristic of the tissue path as follows:

(1) When operated in the Unipolar Mode, the E-Characteristic is theimpedance of the tissue path computed based upon the following equation:##EQU1##

The PathVoltage and PathCurrent are both root mean squared (RMS) values.

In the unipolar mode (see FIG. 8), the voltage is measured between eachtransmitting electrode and the indifferent electrode (or between EI andE.sub.(n), where n represents the location of the current emittingelectrode). The impedance computed by the host processor 206 in thismode reflects not only the impedance of the underlying myocardialtissue, but also includes the impedance of the other tissue mass in thepath. The computed impedance in this mode therefore is not the actualimpedance of the myocardial tissue itself. Rather, it provides arelative scale of impedance (or E-Characteristic) differences of themyocardial tissue lying in contact with the spline electrodes.

(2) When operated in the Bipolar Mode, the E-Characteristic of thetissue is the resistivity of the tissue path, computed as follows:##EQU2## where k is a dimensional constant (in cm) whose value takesinto account the methodology employed (i.e. either Bipolar Two ElectrodeMode or Bipolar Four Electrode Mode) and the geometry of the electrodearray (i.e., the size and spacing of the electrodes).

In general, k is approximately equal to the average cross sectional areaof the current path divided by the distance between the voltage sensingelectrodes. The accuracy of the k value can be further improved, ifdesired, empirically or by modeling.

The PathVoltage and PathCurrent are both root mean squared (RMS) values.

When operated in the Bipolar Two Electrode Mode (see FIG. 9), thevoltage is measured between the two adjacent current emitting/receivingelectrodes (or between E(n) and E(n+1). When operated in the BipolarFour Electrode Mode (see FIG. 10), the voltage is measured between thetwo adjacent electrodes lying in between the current transmittingelectrode and the third adjacent return path electrode (or betweenE(n+1) and E(n+2).

In either Bipolar Mode, the resistivity computed by the processor 206reflects the actual resistivity of the myocardial tissue lying incontact with the spline electrodes. However, the Bipolar Two ElectrodeMode is more prone to electric artifacts than the Bipolar Four ElectrodeMode, such as those due to poor electrical contact between electrode andtissue.

As FIG. 14 shows, the voltage signals sensed by the basket electrodes 38are passed back through the switching element 64 to the data acquisitionsystem 74. As FIG. 11 shows, a signal conditioning element 224preferably corrects alterations to the signal-to-noise ratio occurringin the voltage signals during propagation through the probe body 32.

The data acquisition system 74 includes a multiplexer 76 that selectsand samples in succession the voltage associated with each transmittingelectrode E(n) carried by the basket structure 36. For each selectedcurrent transmitting electrode E(n), the multiplexer 76 samples for aprescribed time period the analog sinusoidal voltage measured betweenthe sensing electrodes.

A sample and hold element 80 stores the sampled analog voltage signals.The stored signals are sent to an analog-to-digital (A-to-D) converter82, which converts the sampled voltage signals to digital signals. Themultiplexer 76 makes possible the use of a single analog-to-digitalconversion path.

The digital signals are sent to a host processor 206 through aninterface 226. The host processor 206, based upon a conventional sortingscheme, obtains the peak voltage and, from that, computes the RMSvoltage. The host processor 206 then computes the E-Characteristic,using the RMS voltage and RMS current (and, for the Bipolar Mode, theconstant k) as described above. The RMS current is known by theprocessor 206, since it has been programmed by it through the interface56 (see FIG. 7).

C. Processing the E-Characteristic

The computed E-Characteristic values can be processed by the system 10in various ways.

In one embodiment (see FIG. 13), the signal processor module includesmeans 90 for sorting the multiple computed E-Characteristic values inabsolute terms, arranging them according to a preassigned electrodenumbering sequence, representing relative electrode position.

The means 90 can create as an output a table (either as a graphicdisplay or in printed form), as follows:

                  TABLE 1                                                         ______________________________________                                        SPLINE       ELECTRODE   E-CHAR                                               ______________________________________                                        S1           E1          75                                                   S1           E2          114                                                  S1           E3          68                                                   S1           E4          81                                                   S2           E1          69                                                   S2           E2          71                                                   S2           E3          67                                                   S2           E4          66                                                   S3           E1          123                                                  S3           E2          147                                                  S3           E3          148                                                  S3           E4          140                                                  . . . etc . . .                                                                            . . . etc . . .                                                                           . . . etc . . .                                      ______________________________________                                    

In Table 1, the spline elements of the basket are identified as S1, S2,S3, etc. The electrodes carried by each spline element are numbered fromthe distal end as El, E2, E3, and so on. The E-Characteristic values areexpressed in terms of resistivity (ohm.cm). The values expressed areidealized and given for illustration purposes. In addition, oralternatively, the means 90 can also create as an output a two or threedimensional display that spatially maps the relative position of thecomputed absolute resistivity values, based upon basket electrodepositions.

FIG. 15 shows a representative display of E-Characteristics (expressedas resistivity values), based upon the data listed in Table 1. In FIG.15, circled Area A identifies a region of low relative tissueresistivity, indicative of infarcted heart tissue. Area B in FIG. 15 isa region of normal tissue resistivity, indicative of healthy hearttissue.

Preferably, the signal processor module 52 also includes means 92 (seeFIG. 13).for arranging the derived absolute E-Characteristics intogroups of equal E-Characteristic values for display in spatial relationto the location of the electrodes 38. This output better aids thephysician in interpreting the E-Characteristics, to identify the regionsof low relative tissue E-Characteristics, where ablation may beappropriate.

As FIG. 16 shows, the means 92 includes a processing step that computesthe location of the electrodes 38 in a three dimensional coordinatesystem. In the illustrated and preferred embodiment, a three dimensionalspherical coordinate system is used.

The means 92 next includes a processing step that generates by computera three dimensional mesh upon the basket surface. The points where themesh intersect are called nodes. Some of the nodes will overlie theelectrodes on the basket. These represent knots, for which the values ofthe E-Characteristic are known.

The values of the E-Characteristic for the remaining nodes of the threedimensional mesh have not been directly measured. Still, these valuescan be interpolated at each remaining node based upon the known valuesat each knot.

One method of doing this interpolation is using three dimensional cubicspline interpolation, although other methods can be used. The cubicspline interpolation process is incorporated in the MATLAB™ program,sold by The MathWorks Incorporated.

The means 92 creates an output display by assigning one distinguishingidicium to the maximum E-Characteristic value (whether actually measuredor interpolated) and another distinguishing idicium to the minimumE-Characteristic value (again, whether actually measured orinterpolated). In the illustrated and preferred embodiment, thedistinguishing indicia are contrasting colors or shades.

The means 92 assigns computer generated intermediate indicia tointermediate measured and interpolated values, based upon a linearscale. In the illustrated and preferred embodiment, the intermediateindicia are color hues between the two contrasting colors or shades.

The means 92 projects the generated color (or selected indicia) map uponthe basket surface, based upon location of the nodes in the threedimensional mesh. The means 92 thus creates as an output a displayshowing iso-E-Characteristic regions.

FIG. 17 shows a representative display of iso-resistivity regions, basedupon the idealized, illustrative data listed in Table 1.

D. Matching E-Characteristic and Tissue Conductivity

FIG. 18 shows another embodiment of a process controller 200 that can beused in association with the probe 18, as already described.

The process controller 200 in FIG. 18, like the process controller 20shown in FIG. 3, includes the current generator module 50 and the signalprocessing module 52 for deriving and processing tissueE-Characteristics in the manners previously discussed.

In addition, the process controller 200 in FIG. 18 includes a module 202for pacing the heart to acquire electrograms in a conventional fashion.The pacing module 202 is electrically coupled to the probe connectors 48to provide a pacing signal to one electrode 38, generatingdepolarization foci at selected sites within the heart. The basketelectrodes 38 also serve to sense the resulting electrical events forthe creation of electrograms.

Operation of the pacing module 202 is not required when ventriculartachycardia (VT) is either purposely induced (e.g., by programmedpacing) or occurs spontaneously. In this situation, the deployed basketelectrodes 38 sense the electrical events associated with VT itself.

The process controller 200 in FIG. 18 further includes a second signalprocessing module 204 for processing the electrogram morphologiesobtained from the basket electrodes 38.

The process controller 200 in FIG. 18 also includes a host processor 206that receives input from the data acquisition system 74 and theelectrogram processing module 204. The processor 206 analyzes the tissueE-Characteristic and electrogram information to compute a matchedfiltered output, which further enhances the CIR of ablation siteidentification.

The modules 202, 204, and 206 may be configured in various ways.

In the illustrated and preferred embodiment (see FIG. 19), the pacingmodule 202 includes a controller interface 208 coupled to the hostprocessor 206, which will be described in greater detail later. Thecontroller interface 208 is also coupled to pulse generator 210 and anoutput stage 212.

The output stage 212 is electrically coupled by supply path 220 andreturn path 218 to the same switching element 64 as the currentgenerator module 50. The switching element 64 has been previouslydescribed and is shown schematically in FIG. 11. As FIG. 11 shows inphantom lines, the pacing module 202 and current generator module 50 areconnected to the switching element 64.

The controller interface 208 includes control buses 214, 216, and 218.Bus 214 conveys pulse period control signals to the pulse generator 210.Bus 216 conveys pulse amplitude control signals to the pulse generator210. Bus 219 constitutes the control bus path for the switching element64.

When used to pace the heart, the switching element 64 distributes thesignals generated by the pacing module 202 to selected basket electrodes38. The pacing sequence is governed by the interface 208, which the hostprocessor 206 controls.

The resulting electrogram signals sensed by the basket electrodes 38 arealso passed back through the switching element 64 to the host processor206 and the processing module 204 through the same analog processingpath as the E-Characteristic signals, as FIG. 11 shows, and as alreadydescribed.

FIG. 20 schematically shows the components of the host processor 206 andthe electrogram processing module 204.

The host central processing unit (CPU) 206 communicates with a massstorage device 230 and an extended static RAM block 232. A userinteractive interface 234 also communicates with the CPU 206.

As FIG. 20 shows, the interactive user interface 234 includes an inputdevice 244 (for example, a key board or mouse) and an output displaydevice 246 (for example, a graphics display monitor or CRT).

The CPU 206 also communicates with the 10 current generator module 50;pacing module 202 and the interface 226 for the system 74, as previouslydescribed. In this way, the CPU 206 coordinates overall controlfunctions for the system 10.

As FIG. 206 shows, the electrogram processing module 204 includes a bus235 and a bus arbiter 236 that receive the digital output of the A-to-Dconverter 82 through the interface 226. The bus arbiter 236 arbitratesthe distribution of the digital electrogram morphology signals to one ormore digital signal processors 238, which also form a part of theprocessing module 204 and which also communicate with the CPU 206.

The illustrated and preferred embodiment employs four signal processors238 operating concurrently, but different numbers of processors 238 canbe used. If N is the total number of basket electrodes and M is thenumber of processors 238, then each processor 238 is responsible forprocessing the signals coming from N/M electrodes in the Unipolar Modeand N/(2M) electrodes in the Bipolar Two or Four Mode.

To speed up data processing, each processor 238 includes a static RAMblock 240. The data is processed real-time and stored in the blocks 240.

The signal processors 238 include various means for processing theelectrogram signals as follows:

(i) to detect the earliest depolarization event;

(ii) to construct from the electrogram signals iso-chronal or iso-delaymaps of the depolarization wavefronts, depending upon how theelectrograms are obtained, which can be presented on the display device246 for viewing by the physician; and

(iii) to construct from the electrogram signals iso-conduction maps,which can also be presented on the display device 246 for viewing by thephysician.

The CPU 206 employs additional means for processing the electrogramsignals and the E-Characteristic signals as follows:

(iv) to match the iso-conduction maps with the iso-E-Characteristicmaps, which can be presented on the display device 246 for viewing bythe physician; and

(v) based upon the matched output of (iv), to identify a potentialablation site.

(i) Identifying the Earliest Depolarization Event

FIG. 21B shows the means 250 for detecting the early depolarizationevent.

The CPU 206 displays the electrograms on the display 246 of theinteractive user interface 234 (see FIG. 21A). After analyzing thedisplay 246, the physician can manually choose a reference time forconventional electrogram beat clustering purposes. The physician can usethe mouse or a keyboard device 244 for this purpose.

In the situation where ventricular tachycardia is purposely induced oris occurring spontaneously, the electrogram beats are clustered relativeto the reference time to compute the propagation time when anelectrogram for ventricular tachycardia is sensed by each electrode 38.For all the beats in the selected cluster, the physician manuallyselects the earliest depolarization event for each electrode 38. Theinteractive interface 234 transmits the physician's choice to the hostCPU 206, which creates a matrix of the computed propagation times.

In the situation where the heart is being paced by the module 202, thebeats are clustered relative to the reference time for computing theactivation delay for each electrogram. The activation delay is measuredbetween the pacing pulse and the earliest depolarization event. For allthe beats in the selected cluster, the physician manually selects theearliest depolarization event for each electrode 38. In this situationas before, the interactive interface 234 transmits the physician'schoice to the host CPU 206, which creates a matrix of the computedactivation delays.

FIG. 21A shows four representative electrograms of a heart undergoingVT. FIG. 21A shows the reference time selected for beat clusteringpurposes and the early depolarization events selected for the purpose ofillustration. From this, the propagation times t₁ ; t₂ ; t₃ ; t₄ can becomputed as the differences between the time of the depolarization eventand the reference time in each electrogram.

(ii) Constructing an Iso-Chronal or Iso-Delay Displays

FIG. 22 shows the means 252 for creating either an iso-chronal displayof the propagation times (when VT is induced or spontaneously occurs) oran iso-delay display of activation times (when the module 202 is used topace the heart). For purposes of description, each will be called the"computed electrogram event."

The means 252 generally follows the same processing steps as the means92 (see FIG. 16) for creating the iso-E-Characteristic display.

The means 252 includes a processing step that computes the location ofthe electrodes in a spherical coordinate system.

The means 252 next generates by computer a three dimensional mesh uponthe basket surface. The points where the mesh intersect are callednodes. Some of the nodes overlie the electrodes on the basket. Theserepresent knots, for which the values of the computed electrogram eventare known.

The values of the computed electrogram event for the remaining nodes ofthe three dimensional mesh have not been directly measured. Still, thesevalues can be interpolated at each remaining node based upon the knownvalues at each knot.

As before, three dimensional cubic spline interpolation can be used,although other methods can be used.

The means 252 creates an output display on the device 246 by assigningone color the maximum value of the computed electrogram event (whetheractually measured or interpolated) and another color to the minimumvalue of computed electrogram event (again, whether actually measured orinterpolated). Computer generated intermediate hues between the twocolors are assigned by the host CPU 206 to intermediate measured andinterpolated values, based upon a linear scale.

The means 252 projects the generated color map upon the basket surface,based upon location of the nodes in the three dimensional mesh.

FIG. 23 shows a representative display generated according to thisprocessing means. The CPU 206 generates this display on the displaydevice 246 for viewing by the physician.

A potential ablation site can be identified at regions where a rapidtransition of hues occurs. Area A on FIG. 23 shows such a region.

When the electrograms used for beat clustering show an induced orspontaneous VT, the resulting display is an iso-chronal map of theexamined tissue region. When the electrograms used for beat clusteringare based upon a paced heart, the display is an iso-delay map of theexamined tissue region.

(iii) Creating Iso-Conduction

FIG. 24 shows the means 254 for creating an iso-conduction displays ofthe computed electrogram event.

An iso-conduction display more rapidly identifies the regions of slowconduction which are candidate ablation sites, than an iso-chronal oriso-delay display. The iso-conduction display requires less subjectiveinterpretation by the physician, as the regions of slow conduction standout in much greater contrast than on an iso-chronal or iso-delaydisplay.

The means 254 draws upon the same input and follows much of the sameprocessing steps as the means 252 just described. The means 254 computesthe location of the electrodes in a spherical coordinate system and thengenerates a three dimensional mesh upon the basket surface. The means254 interpolates the computed electrogram event for the nodes based uponthe known values at the knots.

Unlike the previously described means 252, the means 254 computes theinverse of the magnitude of the spatial gradient of the computedelectrogram event. This inverse spatial gradient represents the value ofthe conduction of the cardiac signal in the examined tissue.

To carry out this processing step, the means 254 first computes thespatial gradient computed electrogram event for each node of the mesh.The methodology for making this computation is well known.

Next, the means 254 computes the magnitude of the spatial gradient,using, for example, known three dimensional vector analysis. Then, themeans 254 computes the inverse of the magnitude, which represents theconduction value.

The means 254 clips all magnitudes larger than a predetermined thresholdvalue, making them equal to the threshold value. This processing stepreduces the effects of inaccuracies that may arise during themathematical approximation process.

The computation of conduction (i.e., the velocity of the propagation)can be exemplified for the case when propagation times are processed. Bysubstituting the activation delays for propagation times, one cancompute the conductions for data obtained from paced hearts.

The location of any point on the three-dimensional mesh shown in FIG. 25is given by the azimuth angle, φ and the elevation angle, δ. The radiusof the underlying surface is normalized to one. The conduction isdefined by EQUATION (1):

EQUATION (1): ##EQU3##

Given that the radius of the meshed surface is one, one obtains thespatial gradient of propagation times:

EQUATION (2): ##EQU4## where Φ and Δ are unity vectors of the sphericalcoordinate system defining the directions of the azimuth and elevation,respectively.

Thus, the conduction can be computed using EQUATION (3):

EQUATION (3): ##EQU5## which is actually the inverse of the spatialgradient magnitude. When the conduction is numerically approximated, thederivatives in EQUATION (3) can be computed by any numerical methodappropriate for the estimation of first derivatives.

The means 254 creates a display by assigning one color the thresholdconduction value (i.e., the maximum permitted value) and another colorto the minimum conduction value. Computer generated hues are assigned tointermediate values, based upon a linear scale, as above described.

The means 254 projects the generated color map upon the basket surface,based upon location of the nodes in the three dimensional mesh.

FIG. 25 shows a representative iso-conduction display generatedaccording to the just described methodology and using the same data asthe iso-chronal display shown in FIG. 23. The CPU 206 generates thisdisplay on the display device 246 for viewing by the physician.

Area A in FIG. 25 shows a region of slow conduction, which appearsgenerally at the same location as the rapid hue transition in FIG. 23(also identified as Area A). FIG. 25 shows the more pronounced contrastof the region that the iso-conduction display provides, when compared tothe iso-chronal display of FIG. 23. Thus, the iso-conduction displayleads to a more certain identification of a potential ablation site.

(iv) Matching Iso-Conduction with Iso-E-Characteristic

FIG. 26 shows the means 256 for matching the iso-conduction with theiso-E-Characteristic for the analyzed heart tissue.

The means 256 derives the values of the E-Characteristic at the nodes ofthree dimensional mesh in the same manner already described. Next, themeans 256 normalizes these E-Characteristic values into an array ofnumbers from 0.0 to 1.0. The number 1.0 is assigned to the absolutelowest E-Characteristic value, and the number 0.0 is assigned to theabsolute highest E-Characteristic value. E-Characteristic values betweenthe absolute lowest and highest values are assigned numbers on a linearscale between the lowest and highest values.

The means 256 also derives the values of the computed electrogram eventat the nodes of three dimensional mesh in the manner already described.The means 256 computes the inverse of the magnitude of spatial gradientof the computed electrogram event, as previously described, to derivethe value of the conduction of the cardiac signal in the examinedtissue.

The means 256 then normalizes these conduction values into an array ofnumbers from 0.0 to 1.0. The number 1.0 is assigned to the absolutelowest conduction value, and the number 0.0 is assigned to the thresholdconduction value. As before, conduction values between the absolutelowest and highest values are assigned numbers on a linear scale betweenthe lowest and highest values.

The means 256 then applies, using known mathematical computationaltechniques, a two dimensional matched filtering process to thenormalized conduction data using the normalized E-Characteristic data asa template, or vice versa. Alternatively, a two dimensionalcross-correlation can be applied to the normalized E-Characteristic andconduction. As used in this Specification, "matching" encompasses bothtwo dimensional matched filtering, two dimensional cross-correlation,and a like digital signal processing techniques.

The values obtained from the matched filtering process are normalized,by dividing each value by the maximum absolute value. Afternormalization, the value will range between 0.0 and 1.0.

The means 256 creates a display by assigning one color the highestnormalized matched filter value and another color to the lowestnormalized matched filter value. Computer generated hues are assigned tointermediate values, based upon a linear scale, as above described.

The means 256 projects the generated color map upon the basket surface,based upon location of the nodes in the three dimensional mesh.

FIG. 27 shows a representative display processed according to the abovemethodology. The CPU 206 generates this display on the display device246 for viewing by the physician.

The display matches the normalized isoconduction values with thenormalized iso-E-Characteristic values, in effect matching electrogramswith tissue E-Characteristics. This matching provides more precisedifferentiation between regions of infarcted tissue and regions ofhealthy tissue.

This information can be further processed to identify a potentialablation site to maximize the CIR.

(v) Identifying a Potential Ablation Site

FIG. 28 shows a means 258 for identifying a potential ablation sitebased upon the matched output of the normalized conduction values andthe normalized E-Characteristic values, generated by the means 256.

The means 258 selects a threshold value. Tissue regions having matchedoutput values above the threshold constitute potential ablation sites.Locating an optimal threshold value can be done by empirical study ormodeling. The threshold value for a given set of data will also dependupon the professional judgment of the physician.

FIG. 29 shows a representative display processed according to the abovemethodology. In FIG. 29, a threshold of 0.8 has been used forillustration purposes. Values greater than the threshold of 0.8 havebeen set to 1.0, while values equal to or less than 0.8 have been set to0.0. The CPU 206 generates this display on the display device 246 forviewing by the physician.

FIG. 29 provides by sharp contrast between black and white (with nointermediate hues) the potential ablation site (Area A).

E. Ablating the Tissue

Regardless of the specific form of the output used, the physiciananalyses one or more of the outputs derived from the basket electrodes38 to locate likely efficacious sites for ablation.

The physician can now takes steps to ablate the myocardial tissue areaslocated by the basket electrodes 38. The physician can accomplish thisresult by using an electrode to thermally destroy myocardial tissue,either by heating or cooling the tissue. Alternatively, the physiciancan inject a chemical substance that destroys myocardial tissue. Thephysician can use other means for destroying myocardial tissue as well.

In the illustrated embodiment (see FIG. 30), an external steerableablating probe 100 is deployed in association with the basket structure36.

Various features of the invention are set forth in the following claims.

We claim:
 1. A system for examining heart tissue comprisingat leastthree spaced apart electrodes, locating means for positioning at leasttwo of the electrodes in contact with a heart tissue region, first meansfor transmitting electrical current in a first path through the hearttissue region between a first pair of the electrodes, for transmittingelectrical current in a second path through the heart tissue regionbetween a second pair of the electrodes, and for deriving therefromtissue electrical characteristics based, at least in part, upon sensingimpedances of tissue lying in the first and second paths, second meansfor sensing timing of local depolarization events in the tissue in whichimpedance is sensed and for deriving therefrom propagation velocities ofthe sensed depolarization events, and third means for matching derivedtissue electrical characteristics with derived propagation velocities inspatial relation to the electrodes to characterize morphology of hearttissue in the heart tissue region.
 2. A system according to claim 1andfurther including means for outputting the match between derived tissueelectrical characteristic and derived propagation velocities in spatialrelation to the electrodes.
 3. A system for examining heart tissuecomprisingat least three spaced apart electrodes, locating means forpositioning at least two of the electrodes in contact with a hearttissue region, first means for transmitting electrical current in afirst path through the heart tissue region between a first pair of theelectrodes, for transmitting electrical current in a second path throughthe heart tissue between a second pair of the electrodes, and forderiving therefrom resistivities of the tissue lying in the first andsecond paths, second means for sensing timing of local depolarizationevents in the tissue in which resistivities is sensed and for derivingtherefrom propagation velocities of the sensed depolarization events,and third means for matching derived tissue resistivities with derivedpropagation velocities in spatial relation to the electrodes tocharacterize morphology of heart tissue in the heart tissue region.
 4. Asystem according to claim 3and further including means for outputtingthe match between derived tissue resistivities and derived propagationvelocities in spatial relation to the electrodes.
 5. A system forexamining heart tissue comprisingat least three spaced apart electrodes,locating means for positioning at least two of the electrodes in contactwith a heart tissue region, first means for transmitting electricalcurrent in a first path through the heart tissue region between a firstpair of the electrodes, for transmitting electrical current in a secondpath through the heart tissue region between a second pair of theelectrodes, and for deriving therefrom tissue electrical characteristicsbased, at least in part, upon sensing impedances of tissue lying in thefirst and second paths, second means for sensing the timing of localdepolarization events in the tissue in which impedance is sensed and forderiving therefrom propagation velocities of the sensed depolarizationevents, and third means for creating matched values that match derivedtissue electrical characteristics with derived propagation velocities inspatial relation to the electrodes to characterize morphology of hearttissue in the heart tissue region, the third means including means foroutputting the matched values in groups of equal matched values inspatial relation to the electrodes.
 6. A system according to claim5wherein the matched values range in value in spatial relation to theelectrodes, and wherein for third means normalizes the value range to anormalized range of 0.0 to 1.0 in spatial relation to the electrodes bydividing each matched value in the value range by a maximum matchedvalue in the value range.
 7. A system according to claim 6wherein thethird means outputs the normalized matched values in groups of equalmatched values in spatial relation to the electrodes.
 8. A system forexamining heart tissue comprisingat least three spaced apart electrodes,locating means for positioning at least two of the electrodes in contactwith a heart tissue region, first means for transmitting electricalcurrent in a first path through the heart tissue region between a firstpair of the electrodes, for transmitting electrical current in a secondpath through heart tissue in the region between a second pair of theelectrodes, and for deriving therefrom tissue electrical characteristicsbased, at least in part, upon sensing impedances of tissue lying in thefirst and second paths, second means for sensing timing of localdepolarization events in the tissue in which impedance is sensed and forderiving therefrom propagation velocities of the sensed depolarizationevents, and third means for normalizing derived tissue electricalcharacteristics in spatial relation to the electrodes, for normalizingderived propagation velocities in spatial relation to the electrodes,and for matching the normalized derived electrical characteristics withthe normalized derived propagation velocities in spatial relation to theelectrodes to characterize morphology of heart tissue in the hearttissue region.
 9. A system according to claim 8wherein the third meansapplies two dimensional matched filtering to the normalized derivedtissue electrical characteristics using the normalized propagationvelocities as a template.
 10. A system according to claim 8wherein thethird means applies two dimensional matched filtering to the normalizedderived propagation velocities using the normalized tissue electricalcharacteristics as a template.
 11. A system according to claim 9 or10wherein the two dimensional matched filtering provides a matchedfilter value range including matched filter values ranging in spatialrelation to the electrodes, the matched filter value range including amaximum matched filter value, and wherein the third means normalizes thematched filter values to a range of 0.0 to 1.0 in spatial relation tothe electrodes by dividing each matched filter value in the matchedfilter value range by the maximum matched filter value in the matchedfilter value range.
 12. A system according to claim 11the third meansincluding means for outputting the normalized matched filter values ingroups of equal matched normalized matched filter values in spatialrelation to the electrodes.
 13. A method for examining heart tissuecomprising the steps oflocating at least three spaced apart electrodesso that at least two of the electrodes are in contact with a hearttissue region, transmitting electrical current in a first path throughthe heart tissue region between a first pair of the electrodes,transmitting electrical current in a second path through heart tissueregion between a second pair of the electrodes, deriving from electricalcurrent transmissions tissue electrical characteristics based, at leastin part, upon sensing impedances of tissue lying in the first and secondpaths, sensing timing of local depolarization events in the tissue inwhich impedance is sensed and deriving therefrom propagation velocitiesof the sensed depolarization events, and matching derived tissueelectrical characteristics with derived propagation velocities inspatial relation to the electrodes to characterize morphology of hearttissue in the heart tissue region.
 14. A method according to claim13wherein the matching step includesnormalizing the derived tissueelectrical characteristics in spatial relation to the electrodes,normalizing the derived propagation velocities in spatial relation tothe electrodes, and matching the normalized derived electricalcharacteristics with the normalized derived propagation velocities inspatial relation to the electrodes.
 15. A method for examining hearttissue comprising the steps oflocating at least three spaced apartelectrodes so that at least two of the electrodes are in contact with aheart tissue region, transmitting electrical current in a first paththrough the heart tissue region between a first pair of the electrodes,transmitting electrical current in a second path through the hearttissue region between a second pair of the electrodes, deriving fromelectric current transmissions resistivities of tissue lying in thefirst and second paths, sensing timing of local depolarization events inthe tissue in which resistivities is sensed and deriving therefrompropagation velocities of the sensed depolarization events, and matchingderived tissue resistivities with derived propagation velocities inspatial relation to the electrodes to characterize morphology of hearttissue in the heart tissue region.
 16. A method according to claim15wherein the matching step includesnormalizing the derived tissueresistivities in spatial relation to the electrodes, normalizing thederived propagation velocities in spatial relation to the electrodes,and matching the normalized derived tissue resistivities with thenormalized derived propagation velocities in spatial relation to theelectrodes.
 17. A method for examining heart tissue comprising the stepsoflocating at least three spaced apart electrodes so that at least twoof the electrodes are contact with a heart tissue region, transmittingelectrical current in a first path through the heart tissue regionbetween a first pair of the electrodes, transmitting electrical currentin a second path through the heart tissue between a second pair of theelectrodes, deriving from electrical current transmissions tissueelectrical characteristics based, at least in part, upon sensingimpedances of tissue lying in the first and second paths, sensing timingof local depolarization events in the tissue in which impedance issensed and deriving therefrom propagation velocities of the senseddepolarization events, and normalizing derived tissue electricalcharacteristics in spatial relation to the electrodes, normalizingderived propagation velocities in spatial relation to the electrodes,matching the normalized derived electrical characteristics with thenormalized derived propagation velocities in spatial relation to theelectrodes to characterize morphology of heart tissue in the hearttissue region by applying two dimensional matched filtering to thenormalized derived tissue electrical characteristics using thenormalized propagation velocities as a template, or vice versa.
 18. Amethod according to claim 17wherein the matching step provides matchedfilter values ranging in spatial relation to the electrodes, and whereinthe matching step further normalizes the matched filter values to arange of 0.0 to 1.0 in spatial relation to the electrodes by dividingeach matched filter value in the matched filter value range by themaximum matched filter value in the matched filter value range.
 19. Amethod according to claim 18and further including the step of outputtingthe normalized matched filter values in groups of equal matchednormalized matched filter values in spatial relation to the electrodes.